During the last two decades, there has been an increasing interest in the utilization of photosensitizers for cancer therapy. In this technique, known as photodynamic therapy (PDT), singlet oxygen, oxygen radicals and superoxides or peroxides are produced by in situ photosensitization of previously applied chromophores and intoxicate the malignant cells (Dougherty, 1987). The technique utilizes non-toxic drugs in combination with non-hazardous photosensitizing irradiation, and has the potential of being more selective yet no less destructive when compared with the commonly used chemotherapy or radiotherapy, and therefore it is expected to increase the quality of life of the treated patients.
The photosensitizers used in PDT need to have a high quantum yield for singlet oxygen production and high affinity and selectivity to the malignant tissue. Porphyrins have a relatively high quantum yield for the formation of an excited triplet state and the difference between the energies of this state and their singlet ground state makes them good energy donors to excite the ground state (triplet) oxygen to its singlet state. It has been known for some time that hematoporphyrin (HP) (FIG. 1) and hematoporphyrin derivative (HPD) tend to accumulate in neoplastic tissues. Thus, HP and HPD mixtures have become the preferred compounds for PDT.
The commercially used HPD mixture as photodynamic agent, Photofrin II (Quarda Logic Technologies, Inc., Vancouver, BC, Canada) contains a high proportion of ether-linked HP oligomers that have high extinction coefficient values around 400 nm (the so-called Soret band), but much smaller values in the visible (500-630 nm; the so-called Q bands). Unfortunately, due to the extensive attenuation of UV-Visible light by the animal tissue, the quantum yield of photosensitization by HPD in situ is very low. Therefore, intensive illumination and large amounts of HPD are required for efficient treatment of tumors. As the amount of applied HPD increases, the chances of its accumulation in normal tissues and the accompanying risk of damaging non-malignant sites, profoundly increases. An additional disadvantage of HPD related analogues is their slow clearance from the human body. Patients treated with HPD suffer from skin phototoxicity over periods of weeks.
The strong attenuation of UV-VIS light by the animal tissue and the limited specificity of HPD to the malignant sites have motivated research and synthesis of new phototherapeutic agents that absorb light beyond 650 nm and have increased retention in the malignant site (McRobert et al., 1989).
In order to increase the retention in, and the specificity to, malignant tissues, various porphyrin derivatives containing particular chemical groups attached to the pyrrole residues of the porphyrin structure have been tested. The red shift of the compounds absorption relative to HPD is achieved by variations in the porphyrin .pi.-electron system.
Following this approach, there has been an increasing interest in using Chl and Bchl derivatives as PDT agents (Kreimer-Birnbaum, 1989; Spike and Bommer, 1991). Chls and Bchls are di- and tetrahydroporphyrin derivatives, respectively, consisting of 4 pyrrole and one isocyclic rings linked to each other and to the atom of Mg, as depicted in FIG. 2 for chlorophyll a (Chla) and in FIG. 3 for bacteriochlorophyll a (Bchla), M representing Mg and the radical R being phytyl or geranylgeranyl in Bchla, and phytyl in Chla. The Bchla molecule differs from the Chla molecule by having two more .beta.-carbon (peripheral carbons of the pyrrole rings) reduced. The variety of Chls and Bchls results from the variation of substituents at the macrocycle or the alcohol residue that esterifies the 17-propionic acid residue. In the naturally occurring Bchla, the alcohol residue is phytyl or geranylgeranyl, while in Bchlb, for example, it is geranylgeranyl. The acids derived from chlorophyll and bacteriochlorophyll are designated chlorophyllide (Chlide) and bacteriochlorophyllide (Bchlide), respectively. The free acids derived from Chla and Bchla are designated Chlidea and Bchlidea, respectively. The compounds derived from Chl and Bchl devoid of a central metal atom are designated pheophytin (Pheo) and bacteriopheophytin (Bpheo), respectively. The pheophytins derived from Chla and Bchla are designated Pheoa and Bpheoa, respectively. The free acids derived from pheophytin and bacteriopheophytin are designated pheophorbide and bacteriopheophorbide, respectively.
The Chls and the Bchls harvest solar energy and initiate electron transfer in biological photosynthesis. Their lowest-energy transitions (the so-called Qz transitions) in the monomeric forms are found at 670-800 nm and can be shifted up to 1000 nm in aggregated forms. These transitions have extremely high extinction coefficients. The probability of inter-system crossing from the excited singlet state of the Chls and Bchls to their lowest triplet state is fairly high (30-50%) and assures a high yield of excited oxygen molecules. In fact, the photosensitization of oxygen by Chls and Bchls is underlined by the fact that they are involved in important degradative processes in photosynthetic bacteria and plants and therefore all photosynthetic organisms have a variety of protective mechanisms against singlet oxygen or oxygen radicals. Since Chls and Bchls are natural compounds that are ordinarily consumed by animals, their in vivo degradation is very fast relative to HP or HPD (Llewellyn, et al. 1990). This is an important advantage that reduces the patient subjection to prolonged irradiation.
Yet, there are several problems in using the native Chl or Bchl extracts for PDT. First, they are hard to deliver to the malignant site because they are strongly hydrophobic. Second, they are very labile under normal delivery conditions, i.e., in the presence of oxygen at room temperature and under normal light conditions. Third, they have no moiety to target them specifically to the malignant tissue. Due to these limitations, the potential of these photosynthetic pigments as sensitizers in PDT is presently hard to realize. It would be highly desirable to synthesize Chl and Bchl derivatives that would overcome these difficulties and could be successfully used in PDT. It is the object of the present invention to provide such derivatives.